docs: def _createCheckAuthRequest(self, message):
          """Generate a check_authentication request message given an
          id_res message.
          """
          signed = message.getArg(OPENID_NS, 'signed')
          if signed:
              for k in signed.split(','):
                  logging.info(k)
                  val = message.getAliasedArg(k)
  
                  # Signed value is missing
                  if val is None:
                      logging.info('Missing signed field %r' % (k,))
                      return None
  
          check_auth_message = message.copy()
          check_auth_message.setArg(OPENID_NS, 'mode', 'check_authentication')
          return check_auth_message